Anycast Transport Protocol For Content Distribution Networks

ABSTRACT

A cache server for providing content includes a processor configured to receive a first datagram from a client system sent to an anycast address, send a response datagram to the client system in response to the first datagram, receive a request datagram from the client system sent to the anycast address, and send a batch of content datagrams to the client system. The first datagram includes a universal resource locator corresponding to the content. The response datagram includes a content identifier for the content. The request datagram includes the content identifier, an offset, and a bandwidth indicator. The batch of content datagrams includes a portion of the content starting at the offset.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to U.S. patent application Ser. No.12/580,861 filed on Oct. 16, 2009, which claims priority to U.S.Provisional Patent Application Ser. No. 61/230,050 filed on Jul. 30,2009, both of which are herein incorporated by reference in theirentireties.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to communications networks, andmore particularly relates to anycast transport protocol for contentdistribution networks.

BACKGROUND

Packet-switched networks, such as networks based on the TCP/IP protocolsuite, can distribute a rich array of digital content to a variety ofclient applications. One popular application is a personal computerbrowser for retrieving documents over the Internet written in theHypertext Markup Language (HTML). Frequently, these documents includeembedded content. Where once the digital content consisted primarily oftext and static images, digital content has grown to include audio andvideo content as well as dynamic content customized for an individualuser.

It is often advantageous when distributing digital content across apacket-switched network to divide the duty of answering content requestsamong a plurality of geographically dispersed servers. For example,popular Web sites on the Internet often provide links to “mirror” sitesthat replicate original content at a number of geographically dispersedlocations. A more recent alternative to mirroring is contentdistribution networks (CDNs) that dynamically redirect content requeststo a cache server situated closer to the client issuing the request.CDNs either co-locate cache servers within Internet Service Providers ordeploy them within their own separate networks.

BRIEF DESCRIPTION OF THE DRAWINGS

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the Figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements. Embodiments incorporatingteachings of the present disclosure are shown and described with respectto the drawings presented herein, in which:

FIG. 1 is a diagram illustrating a communications network in accordancewith one embodiment of the present disclosure;

FIG. 2 is block diagram illustrating an anycast CDN system in accordancewith one embodiment of the present disclosure;

FIG. 3 is a flow diagram illustrating an exemplary method of receivingcontent in accordance with one embodiment of the present disclosure;

FIG. 4 is a flow diagram illustrating an exemplary methods of providingcontent in accordance with one embodiment of the present disclosure;

FIG. 5 is a timing diagram illustrating an exemplary process forproviding content to a client system; and

FIG. 6 is an illustrative embodiment of a general computer system.

The use of the same reference symbols in different drawings indicatessimilar or identical items.

DETAILED DESCRIPTION OF THE DRAWINGS

The numerous innovative teachings of the present application will bedescribed with particular reference to the presently preferred exemplaryembodiments. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily limit any ofthe various claimed inventions. Moreover, some statements may apply tosome inventive features but not to others.

FIG. 1 shows a geographically dispersed network 100, such as theInternet. Network 100 can include routers 102, 104, and 106 thatcommunicate with each other and form an autonomous system (AS) 108. AS108 can connect to other ASs that form network 100 through peeringpoints at routers 102 and 104. Additionally, AS 108 can include clientsystems 110, 112, 114, and 116 connected to respective routers 102, 104,and 106 to access the network 100. Router 102 can provide ingress andegress for client system 110. Similarly, router 104 can provide ingressand egress for client system 112. Router 106 can provide ingress andegress for both of client systems 114 and 116.

AS 108 can further include a Domain Name System (DNS) server 118. DNSserver 118 can translate a human readable hostname, such as www.att.com,into an Internet Protocol (IP) address. For example, client system 110can send a request to resolve a hostname to DNS server 118. DNS server118 can provide client system 110 with an IP address corresponding tothe hostname. DNS server 118 may provide the IP address from a cache ofhostname-IP address pairs or may request the IP address corresponding tothe hostname from an authoritative DNS server for the domain to whichthe hostname belongs.

Client systems 110, 112, 114, and 116 can retrieve information from aserver 120. For example, client system 112 can retrieve a web pageprovided by server 120. Additionally, client system 112 may downloadcontent files, such as graphics, audio, and video content, and programfiles such as software updates, from server 120. The time required forclient system 112 to retrieve the information from the server 120normally is related to the size of the file, the distance theinformation travels, and congestion along the route. Additionally, theload on the server 120 is related to the number of client systems 110,112, 114, and 116 that are actively retrieving information from theserver 120. As such, the resources such as processor, memory, andbandwidth available to the server 120 limit the number of client systems110, 112, 114, and 116 that can simultaneously retrieve information fromthe server 120.

Additionally, the network can include cache servers 122 and 124 thatreplicate content on the server 120 and that can be located more closelywithin the network to the client systems 110, 112, 114, and 116. Cacheserver 122 can link to router 102, and cache server 124 can link torouter 106. Client systems 110, 112, 114, and 116 can be assigned cacheserver 122 or 124 to decrease the time needed to retrieve information,such as by selecting the cache server closer to the particular clientsystem. The network distance between a cache server and client systemcan be determined by network cost and access time. As such, theeffective network distance between the cache server and the clientsystem may be different from the geographic distance.

When assigning cache servers 122 and 124 to client systems 110 through116, the cache server closest to the client can be selected. The closestcache server may be the cache server having a shortest network distance,a lowest network cost, a lowest network latency, a highest linkcapacity, or any combination thereof. Client system 110 can be assignedcache server 122, and client systems 114 and 116 can be assigned tocache server 124. The network costs of assigning client system 112 toeither of cache server 122 or 124 may be substantially identical. Whenthe network costs associated with the link between router 102 and router104 are marginally lower than the network costs associated with the linkbetween router 104 and router 106, client 112 may be assigned to cacheserver 124.

Client system 112 may send a request for information to cache server124. If cache server 124 has the information stored in a cache, it canprovide the information to client system 112. This can decrease thedistance the information travels and reduce the time to retrieve theinformation. Alternatively, when cache server 124 does not have theinformation, it can retrieve the information from server 120 prior toproviding the information to the client system 112. In an embodiment,cache server 124 may attempt to retrieve the information from cacheserver 122 prior to retrieving the information from server 120. Thecache server 124 may retrieve the information from the server 120 onlyonce, reducing the load on server 120 and network 100 such as, forexample, when client system 114 requests the same information.

Cache server 124 can have a cache of a limited size. The addition of newcontent to the cache may require old content to be removed from thecache. The cache may utilize a least recently used (LRU) policy, a leastfrequently used (LFU) policy, or another cache policy known in the art.When the addition of relatively cold or less popular content to thecache causes relatively hot or more popular content to be removed fromthe cache, an additional request for the relatively hot content canincrease the time required to provide the relatively hot content to theclient system, such as client system 114. To maximize the cost and timesavings of providing content from the cache, the most popular contentmay be stored in the cache, while less popular content is retrieved fromserver 120.

FIG. 2 illustrates an anycast CDN system 200 that can be used inconjunction with communications network 100. The anycast CDN system 200can include a CDN provider network 202. The CDN provider network 202 caninclude a plurality of provider edge routers 204 through 214. Theprovider edge routers 204 through 214 can serve as ingress points fortraffic destined for the CDN provider network 202, and egress points fortraffic from the CDN provider network 202 destined for the rest of theInternet. The anycast CDN system 200 can further include cache servers216 and 218. Cache server 216 can receive traffic from the CDN providernetwork 202 through provider edge router 204, and cache server 218 canreceive traffic from the CDN provider network 202 through edge cacherouter 214. In addition to providing CDN service to clients within theCDN provider network, the anycast CDN system 200 can provide CDN serviceto clients within AS 220 and AS 222. AS 220 can include provider edgerouters 224 and 226 with peering connections to provider edge routers206 and 208, respectively. Similarly, AS 222 can include provider edgerouters 228 and 230 with peering connections to provider edge routers210 and 212 respectively. Requests for content from systems withineither AS 220 or AS 222 may enter the CDN provider network through theappropriate peering points and be directed to either cache server 216 or218.

Anycast CDN system 200 can also include a routing control module 232.The routing control module 232 can exchange routes with provider edgerouters 206 through 212 within the CDN provider network 202. As such,the routing control module 232 can influence the routes selected by theprovider edge routers 206 through 212. Additionally, the routing controlmodule 232 can receive load information from cache servers 216 and 218.

Cache servers 216 and 218 can advertise, such as through Border GatewayProtocol (BGP), a shared anycast address to the CDN provider network202, specifically to provider edge routers 204 and 214. Provider edgerouters 204 and 214 can advertise the anycast address to the routingcontrol module 232. The routing control module 232 can provide a routeto the anycast address to each of the provider edge routers 206 though212. Provider edge routers 206 through 212 can direct traffic addressedto the anycast address to either of the cache servers 216 and 218 basedon the routes provided by the routing control module 232. Additionally,the provider edge routers 206 through 212 can advertise the anycastaddress to AS 220 and to AS 222. The routing control module 232 canmanipulate the route provided to provider edge routers 206 through 212based on the load on the cache servers 216 and 218, network bandwidth,network cost, network distance, or any combination thereof. Altering theroute to the anycast address can change which of cache servers 216 and218 serve content to client systems within the CDN provider network 202,AS 220, and AS 222.

In an embodiment, AS 220 may be an unstable network. Traffic from clientsystems within the AS 220 may enter the CDN provider network 202 at bothprovider edge routers 206 and 208. Anycast traffic entering the CDNprovider network 202 at provider edge router 206 may be directed tocache server 216 while anycast traffic entering at provider edge router208 may be directed to cache server 218. Internal routing changes withinAS 220 can cause traffic from a client system within AS 220 to beshifted from cache server 216 to cache server 218, resulting indisruptions to persistent and/or secure connections. As such, it can beundesirable to provide an anycast addresses to client systems within anunstable network that can be subjected to frequent internal routingchanges.

The routing control module can be implemented in hardware, software, orany combination thereof. The routing control module may include one ormore computer systems. When a module includes more than one computersystem, the functions of the module can be distributed across themultiple computer systems in a symmetric manner, such as where eachcomputer system performs the same type of tasks, or in an asymmetricmanner, such as where two computer systems of the module may performdifferent tasks.

FIG. 3 illustrates an exemplary method of receiving content from ananycast aware content delivery network. At 302, a client system can senda datagram to an anycast address for the content delivery network. As aresult of the routing policy, a first cache server responding to theanycast address can receive the request. The datagram can include auniversal resource locator (URL). Additionally, the datagram can be sentusing a connectionless protocol.

Generally, connectionless protocols can provide for sending a messagefrom one system to another without prior arrangement. A sending devicecan send one or more packets or datagrams without first ensuring thereceiving device is available and ready to receive the data.Additionally, connectionless protocols may not define mechanisms tomaintaining a state of the connection or verify the recipient receivedthe message. Using a connectionless protocol, there is no overhead forsetting up a connection and the end-point devices may not need toreserve resources related to connection. Examples of connectionlessprotocols include User Datagram Protocol (UDP), Internet Control MessageProtocol (ICMP), Transparent Interprocess Communication Protocol (TIPC),Internet Package Exchange Protocol (IPX), and the like.

In contrast, a connection-oriented protocol can define a procedure forestablishing a connection prior to sending the message, such as athree-way handshake defined by Transmission Control Protocol (TCP).Additionally, connection-oriented protocols can define mechanisms forverifying delivery of the message, such as an ACK message of TCP, andmaintaining the state of the connection. Once the connection isestablished, the end point devices can be required to track the state ofthe connection and to reserve resources related to the connection untilthe connection is closed or times out.

At 304, the client system can receive a content identification (contentID). The content ID can include an identifier for the contentrepresented by the URL. Additionally, the content ID may include arepresentation of the client system's address. Further, the content IDmay be cryptographically signed or encrypted to reduce the likelihood ofa rogue client system hijacking the content requested by the clientsystem.

At 306, the client system can send a request datagram to the anycastaddress including the content ID, an offset, and a bandwidth indicator.The offset can indicate the beginning of the next portion of thecontent, such as a number of the next byte. The bandwidth indicator caninclude a receive bandwidth of the client for recently received data. Inan embodiment, the first request datagram may not include an offset or abandwidth indicator. In an alternate embodiment, in addition to thecontent ID, the client system may receive a first portion of the contentin response to the datagram containing the URL.

At 308, the client system can determine if a timeout has occurred whilewaiting to receive a portion of the content. If a timeout has occurred,the client system can calculate a recent bandwidth for a portion of thecontent that has been received, as illustrated at 310. At 306, theclient system can send another request for the content, along with anupdated bandwidth indicator and an updated offset. Alternatively, if atimeout has occurred, the bandwidth indicator may be reset to a baserate on the assumption that the timeout was a result of the loss of oneor more datagrams due to a packet buffer overflow at a router along thepath between the cache server and the client system.

Alternatively, when a timeout has not occurred at 308, the client systemcan receive a batch of content datagrams, as illustrated at 312. Thebatch of content datagrams can be limited to a maximum number of contentdatagrams. The maximum number of content datagrams may be a fixed limitor may be a dynamic limit based on network conditions, such as receivebandwidth for recently received data, network congestion, cache serverutilization, frequency of routing changes, or the like. In anembodiment, each content datagram may indicate the number of contentdatagrams within the batch.

At 314, the client system can determine if the content is complete. At316, if the content is complete, the process can end at 316.Alternatively, if the content is not complete, the client system cancalculate the recent bandwidth, as illustrated at 310, and the clientsystem can send another request datagram for a next portion of thecontent, as illustrated at 306. In an embodiment, the bandwidthindicator may be increased by a small amount if all content datagrams ofthe batch were received correctly.

FIG. 4 illustrates an exemplary method of providing content. At 402, acache server can receive a datagram including a URL. The datagram can besent using a connectionless protocol, such as UDP. At 404, the cacheserver can determine a content ID for the content represented by theURL. The content ID can identify the content. Additionally, the contentID may identify the client system. At 406, the cache server can send aresponse datagram including the content ID to the client system.

At 408, the cache server can receive a request datagram from the clientsystem. The request datagram can include the content ID, an offset, anda bandwidth indicator. At 410, the cache server can identify a portionof the requested content based on the content ID, the offset, and amaximum number of content datagrams per batch. The maximum number ofcontent datagrams per batch can be a fixed number or depend on networkconditions, as previously discussed. In an embodiment, the cache servercan verify, based on the content ID, that the request was sent by theclient system rather than a rouge system attempting to hijack thecontent requested by the client system.

At 412, the cache server can send a batch of content datagrams to theclient system including the identified portion of the requested content.The cache server may limit the rate the batch of datagrams is sent basedon the bandwidth indicator. For example, the rate may be not greaterthan the bandwidth indicator. Limiting the rate can be effective toprevent the batch of datagrams from saturating a link on the pathbetween the cache server and the client system. A saturated link mayresult in one or more of the datagrams being dropped when a routeroverflows a packet buffer. Alternatively, the rate may be slightlygreater than the bandwidth indicator to attempt to increase the datarate over time.

In an embodiment, each content datagram may include a few bytes ofbandwidth verification data. The client system can assembly thebandwidth verification data for the batch of content datagrams andprovide the bandwidth verification data or a hash thereof along with thebandwidth indicator in a subsequent request. The cache server can verifythe authenticity of the bandwidth indicator using the bandwidthverification data or hash to ensure that the client system has receivedthe portion of the content.

FIG. 5 is a timing diagram illustrating the communication between aclient system and two cache servers according to an exemplaryembodiment. The communication between the client system and the twocache servers can utilize a connectionless protocol, such as UDP. At502, the client system can send a datagram including a URL to an anycastaddress for the CDN. Using the anycast address, the network can routethe datagram to server A. At 504, server A can send a datagram includinga content ID to the client. The content ID can be an identifier thatuniquely identifies the data file located at the URL.

In an embodiment, the content ID can also include an identifiercorresponding to the address of the server. Additionally, the content IDmay be encrypted and/or cryptographically signed. In another embodiment,server A may also send a batch of content datagrams including a firstportion of the content after receiving the URL datagram.

At 506, the client system can send a request datagram to the anycastaddress which can be routed to server A. The request datagram caninclude the content ID, an offset, and a bandwidth indicator. The offsetcan include a starting byte number of the next portion of the content.The bandwidth indicator can include the receive bandwidth of the clientfor recently received data. At 508, the client system can send a batchof content datagrams including the next portion of the content. The nextportion of the content can begin at the starting byte number indicatedby the offset. Additionally, the number of content datagrams in thebatch can be limited to a maximum number of datagrams per batch.

At 510, after receiving the batch of content datagrams sent by server Aat 508, the client system can send another request datagram to theanycast address. The offset including in the request datagram can beupdated to indicate the next portion of the content begins after therecently received portion of the content. Additionally, the bandwidthindicator can be updated to correspond to the receive bandwidth for therecently received portion of the content. Based on the anycast address,the network can send the request datagram to server A. At 512, server Acan send another batch of content datagrams to the client system.

At 514, after receiving the batch of content datagrams sent by server Aat 512, the client system can send another request datagram to theanycast address. As previously discussed, the offset and the bandwidthindicator can be update. Sometime between the request datagram sent bythe client system at 510 and the request datagram sent by the clientsystem at 514, the network routing may have been changed. A change inthe network routing may not affect the delivery of the content datagramstot eh client system because the content datagrams are addressed to anaddress unique to the client system rather than an anycast address.Accordingly, based on the anycast address, the request datagram can berouted to server B rather than server A. At 516, server B can send abatch of content datagrams to the client system including the nextportion of the content.

FIG. 6 shows an illustrative embodiment of a general computer system600. The computer system 600 can include a set of instructions that canbe executed to cause the computer system to perform any one or more ofthe methods or computer based functions disclosed herein. The computersystem 600 may operate as a standalone device or may be connected, suchas by using a network, to other computer systems or peripheral devices.

In a networked deployment, the computer system may operate in thecapacity of a server or as a client user computer in a server-clientuser network environment, or as a peer computer system in a peer-to-peer(or distributed) network environment. The computer system 600 can alsobe implemented as or incorporated into various devices, such as apersonal computer (PC), a tablet PC, an STB, a personal digitalassistant (PDA), a mobile device, a palmtop computer, a laptop computer,a desktop computer, a communications device, a wireless telephone, aland-line telephone, a control system, a camera, a scanner, a facsimilemachine, a printer, a pager, a personal trusted device, a web appliance,a network router, switch or bridge, or any other machine capable ofexecuting a set of instructions (sequential or otherwise) that specifyactions to be taken by that machine. In a particular embodiment, thecomputer system 600 can be implemented using electronic devices thatprovide voice, video or data communication. Further, while a singlecomputer system 600 is illustrated, the term “system” shall also betaken to include any collection of systems or sub-systems thatindividually or jointly execute a set, or multiple sets, of instructionsto perform one or more computer functions.

The computer system 600 may include a processor 602, such as a centralprocessing unit (CPU), a graphics processing unit (GPU), or both.Moreover, the computer system 600 can include a main memory 604 and astatic memory 606 that can communicate with each other via a bus 608. Asshown, the computer system 600 may further include a video display unit610 such as a liquid crystal display (LCD), an organic light emittingdiode (OLED), a flat panel display, a solid-state display, or a cathoderay tube (CRT). Additionally, the computer system 600 may include aninput device 612 such as a keyboard, and a cursor control device 614such as a mouse. Alternatively, input device 612 and cursor controldevice 614 can be combined in a touchpad or touch sensitive screen. Thecomputer system 600 can also include a disk drive unit 616, a signalgeneration device 618 such as a speaker or remote control, and a networkinterface device 620 to communicate with a network 626. In a particularembodiment, the disk drive unit 616 may include a computer-readablemedium 622 in which one or more sets of instructions 624, such assoftware, can be embedded. Further, the instructions 624 may embody oneor more of the methods or logic as described herein. In a particularembodiment, the instructions 624 may reside completely, or at leastpartially, within the main memory 604, the static memory 606, and/orwithin the processor 602 during execution by the computer system 600.The main memory 604 and the processor 602 also may includecomputer-readable media.

The illustrations of the embodiments described herein are intended toprovide a general understanding of the structure of the variousembodiments. The illustrations are not intended to serve as a completedescription of all of the elements and features of apparatus and systemsthat utilize the structures or methods described herein. Many otherembodiments may be apparent to those of skill in the art upon reviewingthe disclosure. Other embodiments may be utilized and derived from thedisclosure, such that structural and logical substitutions and changesmay be made without departing from the scope of the disclosure.Additionally, the illustrations are merely representational and may notbe drawn to scale. Certain proportions within the illustrations may beexaggerated, while other proportions may be minimized. Accordingly, thedisclosure and the FIGs. are to be regarded as illustrative rather thanrestrictive.

The Abstract of the Disclosure is provided to comply with 37 C.F.R.§1.72(b) and is submitted with the understanding that it will not beused to interpret or limit the scope or meaning of the claims. Inaddition, in the foregoing Detailed Description of the Drawings, variousfeatures may be grouped together or described in a single embodiment forthe purpose of streamlining the disclosure. This disclosure is not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive subject matter may bedirected to less than all of the features of any of the disclosedembodiments. Thus, the following claims are incorporated into theDetailed Description of the Drawings, with each claim standing on itsown as defining separately claimed subject matter.

The above disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments which fall within thetrue spirit and scope of the present disclosed subject matter. Thus, tothe maximum extent allowed by law, the scope of the present disclosedsubject matter is to be determined by the broadest permissibleinterpretation of the following claims and their equivalents, and shallnot be restricted or limited by the foregoing detailed description.

What is claimed is:
 1. A method for providing content comprising:transmitting a response datagram to a client system in response to afirst datagram sent to an anycast internet protocol address by theclient system; receiving, from the client system, a first requestdatagram sent to the anycast internet protocol address, wherein thefirst request datagram includes a content identifier, an offset, and afirst bandwidth indicator; transmitting, by utilizing instructions frommemory that are executed by a processor, a content datagram to theclient system in response to the first request datagram, wherein thecontent datagram includes a portion of the content starting at theoffset; receiving, from the client system, a second request datagramsent to the anycast internet protocol address, wherein the secondrequest datagram includes the content identifier, a new offset, and asecond bandwidth indicator, wherein the new offset indicates a beginningof a next portion of the content that begins after the content datagramtransmitted to the client system, wherein the second bandwidth indicatoris based on a recent bandwidth for the portion of the content from thecontent datagram and based on a timeout.
 2. The method of claim 1,further comprising receiving the first datagram from the client systemsent to the anycast internet protocol address, wherein the firstdatagram includes a universal resource locator corresponding to thecontent.
 3. The method of claim 1, further comprising transmitting anext content datagram to the client system in response to the secondrequest datagram sent to the anycast internet protocol address, whereinthe next content datagram includes the next portion of the contentstarting at the new offset.
 4. The method of claim 1, further comprisingresetting the second bandwidth indicator to a base rate when the timeoutoccurs.
 5. The method of claim 1, further comprising transmitting thecontent datagram to the client system at a rate based on the firstbandwidth indicator.
 6. The method of claim 1, further comprisingverifying, based on the content identifier, that the first requestdatagram was sent by the client system.
 7. The method of claim 1,further comprising verifying an authenticity of the first bandwidthindicator based on bandwidth verification data and a hash of thebandwidth verification data.
 8. The method of claim 1, wherein thecontent identifier includes a representation of an address of the clientsystem.
 9. A system for providing content comprising: a memory thatstores instructions a processor that executes the instructions toperform operations, the operations comprising: transmitting a responsedatagram to a client system in response to a first datagram sent to ananycast internet protocol address by the client system; receiving, fromthe client system, a first request datagram sent to the anycast internetprotocol address, wherein the first request datagram includes a contentidentifier, an offset, and a first bandwidth indicator; transmitting acontent datagram to the client system in response to the first requestdatagram, wherein the content datagram includes a portion of the contentstarting at the offset; receiving, from the client system, a secondrequest datagram sent to the anycast internet protocol address, whereinthe second request datagram includes the content identifier, a newoffset, and a second bandwidth indicator, wherein the new offsetindicates a beginning of a next portion of the content that begins afterthe content datagram transmitted to the client system, wherein thesecond bandwidth indicator is based on a recent bandwidth for theportion of the content from the content datagram and based on a timeout.10. The method of claim 9, wherein the operations further comprisereceiving the first datagram from the client system sent to the anycastinternet protocol address, wherein the first datagram includes auniversal resource locator corresponding to the content.
 11. The methodof claim 9, wherein the operations further comprise transmitting a nextcontent datagram to the client system in response to the second requestdatagram sent to the anycast internet protocol address, wherein the nextcontent datagram includes the next portion of the content starting atthe new offset.
 12. The method of claim 9, wherein the operationsfurther comprise resetting the second bandwidth indicator to a base ratewhen the timeout occurs.
 13. The method of claim 9, wherein theoperations further comprise transmitting the content datagram to theclient system at a rate based on the first bandwidth indicator.
 14. Themethod of claim 9, wherein the operations further comprise verifying,based on the content identifier, that the first request datagram wassent by the client system.
 15. The method of claim 9, wherein theoperations further comprise verifying an authenticity of the firstbandwidth indicator based on bandwidth verification data and a hash ofthe bandwidth verification data.
 16. A method of obtaining content for aclient system comprising: sending a first request datagram to an anycastinternet protocol address, the first request datagram including acontent identifier, an offset, and a first bandwidth indicator, whereinthe content identifier specifies the content to be obtained; receiving acontent datagram from a cache server in response to the first requestdatagram, the content datagram including a portion of the contentspecified by the content identifier and the offset; and sending a secondrequest datagram to the anycast internet protocol address, wherein thesecond request datagram includes the content identifier, a new offset,and a second bandwidth indicator, wherein the new offset indicates anext portion of the content that begins after the content datagram,wherein the second bandwidth indicator is based on a recent bandwidthfor the portion of the content from the content datagram and based on atimeout.
 17. The method of claim 16, further comprising sending a firstdatagram to the anycast internet protocol address, wherein the firstdatagram includes a universal resource locator corresponding to thecontent.
 18. The method of claim 17, further comprising receiving thecontent identifier in response to the first datagram, wherein thecontent identifier includes an address of the client system.
 19. Themethod of claim 16, further comprising receiving a next content datagramin response to the second request datagram sent to the anycast internetprotocol address, wherein the next content datagram includes the nextportion of the content.
 20. The method of claim 16, further comprisingcalculating the recent bandwidth for the portion of the content from thecontent datagram that was received.